1. Field of the Invention
The present invention relates to low-temperature showcases, and more particularly to a low-temperature showcase in which a double air curtain can be formed for a commodity inlet-outlet opening provided in one side of its main body.
2. Related Art Statement
Conventional low-temperature showcases of this type include an open showcase which comprises a case main body having in one side thereof an inlet-outlet opening for commodities and including an inner wall, an outer wall and a partition wall defining between the inner and outer walls an inner passage and an outer passage for passing air therethrough, two heat exchangers disposed in the inner and outer passages respectively for providing refrigeration cycles along with a compressor, condenser and reducing valves, and two blowers disposed in the inner and outer passages respectively for passing air through the two passages in the same direction, so that at least a double air curtain can be formed for the opening with the air circulated through the inner and outer passages.
Among low-temperature showcases of this type, that disclosed in the specification and the accompanying drawings in U.S. Pat. No. 4,648,247 and that presented as a freezer disclosed in Japanese Patent Publication No. 58082/1988 have a common construction in which a heat exchanger and a blower are disposed in each of inner and outer passages respectively, and air curtains are made alongside of each other in an opening by air circulated through the passages during refrigeration operation of the inner heat exchanger. Further in the showcases, the outer heat exchanger is disposed downstream of the inner heat exchanger with regard to flow of the circulated air, a partition plate between outer and inner walls defines the inner and outer passages, the partition wall is provided with a window between the inner and outer heat exchangers so that the outer and inner passages communicate with each other through the window and also provided with a damper movable for opening and closing the window, which is in the opening position during refrigeration operation of the outer heat exchanger. The inner heat exchanger, when operated for defrosting, is forcibly heated with a refrigerant such as a hot gas, a liquid refrigerant and a gas-liquid mixed refrigerant, serving as a heat source for defrosting. In this way, frost built up on the inner heat exchanger is removed.
In accordance with the aforementioned prior art, when the inner heat exchanger functions as an evaporator for refrigeration, a cold air flowing across the opening is below the freezing point in temperature, and also it is kept below the freezing point in temperature while flowing through the inner passage to return to the inner heat exchanger after the crossing of the opening. As a result, the surface of a drain receiver usually formed on the bottom of the inner passage is kept below the freezing point in temperature.
When the refrigeration operation ends and defrosting operation starts in the inner heat exchanger, the inner heat exchanger is forcibly heated with a refrigerant serving as a heat source for defrosting. Consequently, frost built up on the inner heat exchanger gradually melts into pieces of ice and/or drain water to fall down on the bottom of the drain receiver. The surface of the drain receiver is kept below the freezing point in temperature as has been described. Further, air heated in the inner heat exchanger during the defrosting operation of the inner heat exchanger flows through the window into the outer heat exchanger serving as an evaporator and is subjected to heat exchange to be cooled, and thereafter the cooled air in the inner passage is kept at about 0.degree. C. in temperature from the middle of the defrosting operation to the latter period thereof although it experiences a slight increase in temperature when flowing across the opening. Accordingly, it takes a longer period of time for the temperature of the drain receiver to rise to 0.degree. C. or over, and hence it takes a great deal of time for pieces of ice falling down on the drain receiver to melt. Also, the drain receiver is ill drained due to the pieces of ice. This causes the pieces of ice to gradually grow into blocks of ice and also causes drain water to be frozen into an ice sheet. These ice blocks and ice sheet impedes the passage of a circulated air, so that the flow rate and flow velocity of the air curtain are reduced, and frozen load in the opening is increased.